Rescue SMTP errors when sending suspension mailers

also, ensuring we retry only on errors where
it makes sense to retry: like timeouts and
server busy errors.

related reading:

http://robots.thoughtbot.com/i-accidentally-the-whole-smtp-exception
http://ruby-doc.org/stdlib-2.0/libdoc/net/smtp/rdoc/Net/SMTP.html#method-c-start-label-Errors
the first rescue block is necessary because we need to rescue and retry. wondering if we need a second rescue block, because it only logs a warning, airbrake will be notified for all exceptions anyways. should i get rid of it ?